US2486620A

US2486620A - Antenna system for short waves

Info

Publication number: US2486620A
Application number: US507585A
Authority: US
Inventors: Lester C Van Atta
Original assignee: US Department of Navy
Current assignee: US Department of Navy
Priority date: 1943-10-25
Filing date: 1943-10-25
Publication date: 1949-11-01
Anticipated expiration: 1966-11-01

Description

2-SheesSheet 1 INVENTOR Nov. l, 1949. L. c. VAN ATTA.

ANTENNA SYSTEM FOR SHORT WAVES Filed Ooi. 25, 1943 L. c. VAN ATTA y 2,486,620

ANTENNA SYSTEM FOR SHORT WAVES N ov. l, 1949.7

Filed Oct. 25, 1 945 2 Sheets-Sheet 2 INVENTOR LESTER C. VAN

ATTA

Patented Nov. 1.949g

Lester C. Van. Attaj WinchesterMass.- ,assigner, by mesne assignments, to the United States'of` America as represented by. the Secretary of the Navy Application Octoberf 25, 1943-, SerialNoz 507,585

15 claims. l

This invention relates to radio antenna systems, andK particularly antenna systems for operation on micro wave lengths, including an antenna, a smalslfreecting element located. near and forwardly of said antenna and a large reflector at a greater distance from said antenna and locatedl rearwardly of it.

Directive antenna systems for micro wave lengths, which is to say for wave lengths less than about 50 centimeters, have in the past4 made greatv use of parabolic reflectors` which are able to give a high degree of directivity. Parabolic reflectors are also fairly readily adapted for aiming` in different directions and may when suitably mounted be moved about rapidly and convenientlyI without appreciable distortion of the directivity of the beam. produced. In order to realize the full advantage of a. parabolic reiiector it is important that the paraboloid be properly illuminated at all times,- which isto say that the antenna system associated with the parabolic reflector should radiate as much as possible in the direction of the paraboloid and as little as possible in other directions, especially directions at wide angles to the axis of the parabolic reflector.

A- metallic sheet reflector in. front of the antenna is sometimes used for the purpose of cutting off all direct forward radiation of the antenna and reflecting such radiations back into the parabolic reflector. Various shapes of sheet metal reflectors have been used. A circular disk somewhat less than one wave length in diameter has been found to be the most effective and suitable of such arrangements. be placed approximately a quarterewave length in front of the antenna, and preferably slightly more than a quarter-wave length, for instance about 0.27 wave length. In such an arrangement the effective center of radiation of a combination of a dipole antenna and a reflecting disk usually lies midway between the dipole antenna and the reflecting disk. In some cases ity may lie closer to the reflecting disk than such' inidpoint. The location of the effective center of radiation can be determined experimentally with sufficient accuracy for antenna design information. Such determination may make use of measurements and plots of phase patterns in the neighborhood of the system, from which the location of the effective center can be deduced.

An ultimate object of this invention is torv provide a steerable directive antenna system for microwave lengths providing a more concentrated and powerful beam than has heretofore Such a disk should f 2.. been possibile for` a given transmitter power and paraboloidreflector. dia-meten Particular gobe'cts of thisinventionv includef. production of an antenna` with an: improved radiation pattern (with regard tof side. lobes, beam. width, etc.), the provision;y of an4 antenna which is` properly matched toA the. feed: line associated therewith sothattherefwill be maximum transfer of energy to: the transmitted.` beam, and the provisionpf means for. illuminating a parabolic reectoiwith microwavefradiatiommore effectively than in the previous. practica.

Thev invention is illustrated. in; the accompanying4v drawings in: which:

Eig.;` 1 is a side; View;` partly in section,l showing'` the preferrediformf of antenna systems in` accordance with= this: invention;.v

Fig. 2.isI a sectionalview showing in detail the construct-ion of the forward part ofthe antenna system of Fig..y 1;.

Fig.. 3. a. perspective view of another form ofi antenna'. system. according to this invention, the parabolic; reflector. being omitted for con,- venience, and

Figs; 4,. 5,1. and' 6k are diagrams ofV radiation patternsfthatzare believed to correspondv to certain: antenna. systemsherein described andi which are providedifor elucidation' of the principlesberlievectl to control; the operationsv of antenna sys'- tems. according. to.y this invention.

I have' found. that highlyy satisfactory illumination. of. parabolic relectorsffor the. purpose of forming a concentrated bearrr of radiation can be obtained witlr ai parasitic dipoler reflector, sometimes known as` a: dummy.,-. infront of' the an:- tenna properinstead off a s'heet or disk reflector provided the' dummy is:4 properly spaced A though fair concentration ofl the: radiated beam can be achieved with such; a dipole located one quarter-wave: length ini frontl of the antenna dipole; the directionalproperties-of the beam are greatly improved if the parasitic reflector dipole is spaced' considerably closery to the antenna proper: than; one quarter-wave length; and.l preferably oneeig'htliewave length. This is. especia-ily true if: at the samej time the distancek between th'e midpoint of the antenna-parasitic'- 3 in the drawing the parasitic reflector 3 is supported on a projection 4 inside a thin bulb 4a of high quality insulating material, such as polystyrene, which serves to exclude moisture and dust and may also serve to maintain atmospheric or higher pressure to prevent the occurrence of corona discharges when the apparatus is used at high altitudes. The antenna dipole I, 2 is fed by a coaxial feed line 5 and is provided with rounded arms I and 2, these arms being rounded primarily to reduce corona. It will be noted that the dipole I, 2 is at right angles to the axis of the feed line 5. This configuration is preferred because it permits introducing the feed line through the vertex of the parabolic reflector and supporting the feed line and dipoles at the said vertex, thus keeping the feed line symmetrical with respect to the parabolic reflector, which is a great advantage in obtaining a concentrated beam of radiation and a good directional radiation pattern. If for some reason the advantages resulting from the symmetrical location of the feed line with respect to the parabolic reflector can be dispensed with, other advantages of this invention can of course still be realized with other well-known arrangements of the feed line with respect to the dipoles and the parabolic reflector.

In order to provide better support of the feed line and antenna system from the vertex of the parabolic reflector the outer conductor of the feed line 5, as shown on Fig. 1, is progressively thickened as it approaches the parabolic reflector, which is shown at 6, so that the outer surface of the feed line 5 has a flaring shape. The feed line and its associated antenna system is fastened to the parabolic reflector 6 by a suitable clamping or bclting device, a simple form of which is shown on Fig. l in the form of a shaped washer 8 and a nut 9 cooperating with a shoulder Ill forming part of the feed line body; or the feed line may enter through a slot or hole in the reflector when the latter is to be moved with relation to the antenna proper. The end of the feed line on the outside of the parabolic reflector 6 is shown threaded at I I for receiving a coupling for the purpose of connecting it to a further section of line or a rotating joint forming part of the transmission and reception system with which the antenna system is designed to be employed.

The effective center of radiation of the system comprising the dipole I, 2 together with its parasitic reflector dipole 3 is the midpoint between the centers of the respective dipoles. Heretofore it has been customary to locate the effective center of the illuminating source at the focus of the parabolic reflector for the formation of a directive beam of radiation, irrespective of the relation between the focal length and the wave length in question. I have found that it is more important to locate the effective center of radiation at a point separated from the vertex of the parabolic reflector by an integral number of half-wave lengths than to take care to locate the effective center of radiation exactly at the focus. I therefore preferably locate the effective center of radiation at that number of half-wave lengths from the vertex of the parabolic reflector which brings the effective center of radiation nearest to the focus, which is to say that in the apparatus of Fig. 1 the dimension a should be equal to where i is the Wave length and' 11. is the whole number which brings the expression nearest to the focal length of the parabolic reflector.

In cases in which the focus falls approximately half-way between two half-wave points, the effective center of feed should be located at that halfwave point which more nearly corresponds to the subtending of the optimum solid angle at the effective center by the aperture of the parabolic reflector. This will depend on the directivity of the feed (the feed being the antenna dipole and auxiliary reflector combination) and on the shape (i. e. focal-length-diameter ratio) of the parabolic reflector. For example, if the parabolic reflector is initially somewhat too deep, the first half-wave point beyond the focus should be chosen. The matter of the most desirable paraboloid shape for antennas of this invention is discussed further below. In any event, of the points at integral numbers of half-wave lengths from the vertex of the parabolic reflectors, one of the two falling closest to the focus is generally to be chosen for the effective center of radiation. If this is done, such effective center will in every case be not more than a half-wave length from the focus. If the nearest point is chosen, the effective center will be at most a quarter wave length from the focus.

Where it is possible to do so. I provide the antenna system with a parabolic reflector the focal length of which is equal to an integral number of half-wave lengths of the radiation in question.

,It is not always convenient to do so, however,

because templates for the manufacture of parabolic reflectors are expensive and it is consequently not economical to manufacture such reflectors in many different focal lengths. The adjustment just described of the dimension a, referring to Fig. 1, is especially important where close spacing such as spacing of the order of one eighth wave length, is used between the antenna dipole I, 2 and the reflector dipole 3, for reasons more fully explained below.

As is more fully explained below in connection with Figs. 4, 5 and 6, the importance of the above considerations in locating the antenna and its associated auxiliary reflector with respect to the paraboloid vertex depends on two factors: (a) the proper phasing of direct forward radiation with radiation reflected from the parabolic reflector and (b) the obtaining of a minimum antenna input impedance in order to match the feed line as well as possible to the antenna. It is important that the input impedance of the antenna system be kept low in order that this impedance may be capable of being matched to the feed line by means of a sleeve type transfoarmer of practicable dimensions. If the transformer is required to provide a large transformation ratio, excessive frequency-sensitivity may result. When a given system, consisting of a feed line, an antenna and an auxiliary reflector, is once adjusted for impedance match at a point near the focus of a parabolic reflector located on the axis of the latter and an integral number of half-wave lengths from the vertex, it is substantially matched for other such half-wave point locations of the same parabolic reflector and also substantially matched, assuming a standing wave power ratio of 1.5 can be tolerated, for such half-wave point locations in other parabolic reflectors, irrespective of the focal length of the latter, provided that the original match was obtained in a reflector having a focal length of 1.5 wave lengths or more and that the other reflectors in question also have focal lengths of at least 1.5 wave lengths. For smaller reflectors, the match depends on the focal length. For the large reflectors, the possibility of`4 change of' reflectors without rematching holds also for an-v tenna feeds matched fory locations other than the half-wavepoints, provided the location relative to the half-wave point is maintained when the reflector is changed.

llhus by the use of the relations here disclosed an antenna feed may be designed for effectively illuminating any of a Wide variety of beam-forming parabolic reflectors.

Further features of the preferred form of antenna are shown in detail on Fig, 2. As illustrated in that ligure a detachable coupling is provided in the feed line 5. at some distance in back of the antenna in order that the antenna may be replaced or removed for adjustment withoutrunfastening the feed line 5 from its support at the vertex of the parabolic reector (not shown on Fig. 2). The coupling includes a threaded tightening sleeve l5, a threaded bushing llmounted on the outer conductor il of the eed line 5, a gasket i8 made of a compressible elastic material such as neoprene, a split steel friction ring i9 and a connector plug 2f! fitting intoA recesses 2l and 22.` in the respective

inner conductors

23 and 24 of the two portions of the feed line. Insulating beads or spacers 25 are shown in the feed line 5 for maintaining the relative positions of the inner conductor 23 and the outer conductor l1. 2G forms the outer conductor of the section of coaxial line between the coupling just described and the antenna. It may be conveniently made of machined brass. It is provided with a shoulder 2l and flange 28 which serve respec- 1" tively for mounting, a choke sleeve 29 and for holding in place the inner member or cup 33 of the polystyrene protective bulb about the antenna system. A gasket 3|v of compressible material is provided between the flange 28 and the l' cup 3l) for the formation of an airtight seal between the polystyrene bulb and the tubular conductor 26. A metallic sleeve or cup 32 is threaded on the outside of the tubular conductor 26 so that its mouth ts over the outside of the neck f of the cup 3G, thus holding the cup 30 firmly in placel and at the same time providing a radio frequency choke adapted to inhibit the formation of standing waves on the feed line 5. A

lock nut 33 is provided for holding the cup 32 dimension b on Fig. 2) must be carefully adjusted. This length should be such that a quarter-wave length oscillation can be maintained in the cavity formed by the conducting parts, which is to say that the cavity acts as a closed pipe type of resonator. the polystyrene insulation the length of the dimension b is considerably less than the quarterwave length of oscillations of the frequency in question radiating in the open air. It can however be calculated by known means from the dimensions of the structure and the characteristics of the dielectric, or it may be determined experimentally. A more specific value of this dimension is given below together with values of certain other dimensions which have been found suitable for a particular range of wave lengths in an apparatus of the particular configuration of Fig. 2. If the apparatus is to be used at high altitude where the air surrounding the antenna system on the outside of the cup 30 is at a pres- The tubular element e Because of the presence of i:

sure conslderably.belowy normal atmosphericrpresf-f sure, it maybeadvisabl'elto. cement the` cup130 to the structure 32 wheny the apparatus is as! sembledf to: prevent. the occurrence' of corona. in theresonator formed: by thestructure 32 onaccount of theioccurrenceof" narrow air spaces ins side the structure 32.

The'sleeve- 29,1, like the structure 32, cooperates with the tubular conductor 26 to` provide ares.- onatoronI the outside of the conductor 26' which acts to prevent the occurrence of standing: Waves on the outside ofthe conductor 26 and generallyr on` the feed line 5'. This resonator isagain.` a quarter-wave resonatorwith its open. end directed towards the.A antenna. The depth of. the. cavity should again. correspond to an. electrical quarter. wave length, which in. practice is slightly less than a quarter of the- Wave length in open4 air of radiation of corresponding frequency, the variation from the` exact quarter-wave length. being probably due toend effects in. thecavity;

The resonator formed: bythe sleeve 29 maybe regarded as a choke to suppressstanding waves on the outside ofy the feedl line or itmaybe considered as a balancing resonator providing a transitionv (i. e. acting as av transf-ormer, in a sense)` between the unbalancedy rear portion feed line (one conductor being at R1 F; ground po.- tential) and the balanced portion feed` linel near the antenna where both conductors haver R. F. voltages. withy respect to. ground'. In` this sense the resonator permits the forwardy end portion of the line 5i tobehave as a balanced line While theA rear portion remains unbalanced as required for normal operation. The distancebetween the mouth of the choke sleeve 2a and the antenna l, 2 is quite important since it affects the impedance offered by the antenna system to the feed line. In general the choke should be close to the antenna, preferably considerablyl less than a quarter-wave length, and as close as the danger of corona discharges will permit under the desired conditions of operation. The exact spac' ing is not quiteA so critical for small differences in wave length as some of the other distances here mentioned and the effect of this spacing upon the antenna impedance may be compensated for or taken into account in selecting the proportions for the quarter-wave matching transformer provided as hereinafter described on the inner conductor 24.

The dipole arm- I, formed in a rather bulbous shape for the mitigation of corona and for the reductionof frequency sensitivity, is mounted directlyy on the end of thel tubular conductor 26 as shown, the tubular conductor 2E being cut away on the opposite side at an angle of about 35 (see also` Fig. 1) in order to allow ample clearance for the dipole arm 2, which is fastened upon a rounded end piece 35 threaded on the end ofthe inner conductor 24 of the feed line in such a manner as to embrace the conical insulating bead 36. The dipole arm 2 is preferably of the same shape and size as the dipole arm l, in order to provide as even a configuration of the potential gradient as possible and thereby inhibit corona discharges. The electrical center of the dipole antenna I, 2 is not exactly on the axis of the feed linel 5, so that it is advisable to align the reflector dipole 3 slightly olf-center with respect to the axis of the feed line 5 so that it may be symmetrical with respect to the midpoint of the dipole I, 2 as shown on Fig. 2. It is not certain that this slight asymmetry results in a similar asymmetry of the effective center of radiation,

because the latter is also affected slightly by the` presence of standing waves between the mouth of the choke sleeve 29 and the antenna, which standing waves will-in general not be symmetrical 'with respect to the axis of the feed line, producing an effect which may well outweigh as well as counteract that produced by the mechanical displacement of the dipole with respect to the axis of the feed line. Thus for all practical purposes the antenna system shown in Fig. 2 acts as if its f effective center of radiation were on the axis of the feed line and halfway between the axes of the dipole I, 2 and the dipole 3. The length of the dipole 3 should approximate an electrical half-wave length and is preferably somewhat longer than the dipole I, 2. The preferred physical dimensions, in terms of the free space wave length, are given in the table below.

Keying arrangements 3i' are provided for maintaining the insulating bead 36, which is preferably made of high quality polystyrene, in fixed relation to the inner and outer conductors of the feed line. The bead 36 is also preferably provided with a longitudinal groove (not shown) at some point on its outer periphery for the purpose of allowing. communication between the atmosphere in the feed line and the atmosphere in the bulb or pressure vessel surrounding the dipoles. Thus at the same time the antenna system and the feed line may be maintained at atmospheric or superatmospheric pressure for the prevention of sparking and corona. It is to be noted that the end piece 35 is provided with a rounded end surface the shape of which contributes to the prevention of corona discharges.

The inner conductor 24 which is connected to the dipole arm 2 through the conducting end pie-ce 35 is provided with a thickened portion 38, the length of which is shown on Fig. 2 by the dimension d. This length is usually approximately a quarter-Wave length of the oscillations in question, although other lengths may be used provided a suitable coordination of the Spacing between the thickened portion and the antenna is made in accordance with principles well known in the art. The amount by which the diameter of the inner conductor 24 is increased for the length of the thickened section 38 depends not only upon the dimensions of the antenna I, 2 and the position of the choke sleeve 2Q but also rather considerably` upon the distance between the thickened portion 38 and the antenna. The desirable distance between the thickened portion 38 and the antenna will be affected somewhat by the nature and configuration of the insulating bead 3S. With proper adjustment of this length (the dimension e on Fig. 2) the necessary thickening of the quarter-wave matching transformer 38 may be very small, such as one hundredth of an inch additional radius. In general it sho-uld .be noted that the antenna impedance is affected by variations in spacing between the antenna dipole and the reflector dipole and also by nearby reflecting objects in the path of the beam cast by the parabolic reflector, so that the matching transformer 38 should be designed with regard to the antenna impedance as it .occurs in the service position of the antenna system. The antenna system of Figs. l and 2 constructed as herein described and mounted without obstructions in the path of the beam cast by the parabolic reflector has an input impedance of not very much greater than the characteristic im.. pedance of the feed line 5 which may be about ,50 ohms, so that only a small thickening of the inner conductor 24 will suice to provide a suitable impedance match.

In an apparatus of the particular type shown in which apparatus the inner diameter of the outer feed line conductor was between 1/ and wave length, good results have been obtained with the distance e almost exactly equal to a quarter of the free-space wave length. The electrical wave length in question is greater and may be nearer a half than a quarter of the wave length on account of the insulating bead 36. The antenna input impedance at the point of feed is slightly inductive in the particular apparatus shown in Figs. 1 and 2 when constructed as herein described for wave lengths between 9 and 10 centimeters, Ibut this effect is to some extent compensated by the capacitive loading resulting from the location of the insulating lbead 36 between the inner and outer conductor.

The design of matching transformers consisting of a quarter-Wave section of transmission line of a characteristic impedance different from that of the line to which it is desired to match a particular load impedance is well understood. The determination of the impedance of the load is not always so simple, particularly at microwave frequencies. Techniques have recently been evolved however for making this determination, and one of the most practical of these makes use of the measurement of the standing wave ratio resulting from the connecting lof the load impedances in question to a line of known characteristic impedance. Such a determination will provide a basis for calculating the desired dimensions of matching transformer and its location in the line. The accuracy of the calculation can then -be checked by another measurement of standing waves with the transformer in the line. The nature of the error will indicate the proper direction for correction of the dimensions of the transformer.

In general, the characteristic impedance of the quarter-wave matching section of line should be equal to the geometric mean of the Icharacteristic impedances of the line and of the load, as is well known in the art. The desired characteristic impedance may be obtained in the quarter-wave section Iby varying the dimensions of the conductors or the dielectric constant of the medium (i. e., -by varying the medium) between them, or both. The characteristic impedance' Zo of a coaxial line depends on the' crosssectional dimensions of the line as follows:

where b is the inner diameter of the outer conductor and a is the outer diameter of the inner conductor.

The polystyrene cup 3D is closed by another cup-like mem-ber 40 preferably made lof the same material to form an airtight vessel or bulb about the dipoles. The

cups

30 and 40 are put together by a close-fitting joint I where they are preferably cemented together -by polystyrene cement. Such a cemented joint will hold at all reasonable pressure differentials between the inside and outside of the bul-b. No observable effect on the radiation pattern of the antenna system has yet been traced to the presence of the polystyrene enclosure on antenna systems constructed in accordance with Figs. l and 2 with reasonably thin bulbs. The cup 40 is provided with a central protuberance 42 upon which is mounted a meri ber 43made of similar insulating material which is adapted to hold in place the reflector dipole 3 in cooperation with a kingpin 44. The member 43 is adjusted and dimensioned suitably for holding the reflector dipole 3 at the proper spacing from the antenna dipole I, 2 for the wave length in question.

In order to give complete details concerning the preferred form of antenna for the purpose of illustrating more fully the way in which the invention is best carried out there are tabulated below certain dimensions relating to the apparatus of Fig. 2 which have been found suitable for operation at wave lengths in the microwave range. When variation of one or more of these dimensions is contemplated it should be remembered that these dimensions are in general interdependent and the performance with respect to the other dimensions should be checked if one of these dimensions is varied substantially. The dimensions given below relating to the matching transformer 38 have been experimentally checked only for la narrow range `of shorter wave lengths, but values for other wave lengths in the microwave range are not likely to be much different, in terms of the wave length, for a similar antenna structure. Since it is convenient within the aforementioned range of wave lengths to keep constant the position of the mouth of the choke sleeve 29, changes in wave length of operation and corresponding changes of the dimensions given below in accordance with wave lengths may require a further adjustment of the matching transformer diameter and location because ofv a change of antenna impedance. The dimensions given below are expressed in terms o-f the free space wave lengths :and of course apply only if substantial changes in the configuration of spacing insulators such as the bead 36 and the like are avoided. Where the dimension given is a range of values, the first value given represents a value which has been found successful at shorter wave lengths, and; the dimension last given represents a value that has been found successful at longer wave lengths. The values relating to the matching transformer were obtained with the former values of the other dimensions. The variations are at least partly explained in that the feed line diameters and the choke spacing were held constant while the other dimensions and the frequency were varied.

Table lndlca- M agnitude' in Dimension tion on Free Space Fig. 2 Wave Lengths Depth of polystyrene-filled choke b 0.128 Depth of inner choke c 0. 23 Length of'matching transformer d 0.248 Position of matching transforme e 0. 249 Length of parasitic dipole f 0. 45 to 0. 44 Length of antenna dipole 0.408 Dipole arms h 0.161 to 0.167 Spacing between dipoles 1 0.122 to 0.127 I. d. of outer conductor of Ieedline 0.157 to 0.134 O; d. of inner conductor of feed line-. 0:070 to 0. 059 Diameter of matchingtransformer 0. 084

Fig. 3 shows another form of antenna according to this invention which is suitable for use under conditions in which the protective polystyrene bulb shown in Figs. 1 and 2 may be dispenseel with. As in Fig. 2, only the forward part of the antenna system is shown, the parabolic reflector being omitted for convenience of illustration. In this form of the invention the reflector dipole 3is carried.- ona yoke,` 50,.' which forms` an extension of a tubular sleeve 5I which may be slidably adjusted longitudinally along the outside of the choke sleeve 29. The position of sleeve 5| and its yoke extension 50 may be fixed by means of a set screw 5.2. The yoke 50 andthe sleeve 5| may conveniently be made of conducting material, such las brass, for since the arms of the yoke 5!! are in a plane perpendicular to the dipoles, the structure does not interfere with the operation of the choke sleeve 29. This structure, however, may also be formed of insulating material. Except for the omission of the polystyrene protective buib and its associated parts and for the substitution of the structure 50, 5I for the support of the parasitic` dipole 3, the construction of the antenna system shown in Fig. 3 preferably follows that of. the antenna system shown in Figs. l and 2.

The rounded. end of the end piece of the inner conductor 35 of Fig. 2 appears at 53 on Fig. 3. Likewise atti is shown part of the surface of the insulating bead shown in cross section at 36 on Fig. 2.

Although it has been determined by experiment that the use of close spacing of the order of one-eighth-wave length between the antenna dipole and the reflector dipole, instead of onequarter-wave length spacing previously thought to be most desirable, results in better directive patterns for the entire system including a parabolic reflector, it is rather diicult to explain on theoretical grounds why the closer spacing should be superior or even to give an accurate description of just how the parabolic reflector is illuminated by the combination of close-spaced dipoles. It is to be noted at the outset that the requirements of an antenna system for illuminating a parabolic reflector and producing therewith a highly directive beam of radiation are to a large extent determined by the maximum allowable diameter for the parabolic reflector. In a steerable antenna system. and particularly in one which is regularly moved about for purposes of scanning, it becomes rather diiiicult to handle very large parabolic reliectors and this difculty is all the greater when the antenna system is to be mounted on seagoing vessels or on aircraft. Consequently the most practical sizes of parabolic reflectors are in the range between` 5I and 15 wave lengths,v with reflectors with a diameter of about 8 wave lengths beine rather commonly used.

The shape most desirablel fm` the parabolic reector depends to some extent on the directive properties of the feed. Thus, although before this invention it was common practice to use a .parabolic reiiector with a focal length equal to about 0.35 times the diameter, with tbeftype of feed which I prefer, havingr a dipole auxiliary reector about 1A; wave length in front of the antenna dipole. I nd that a parabolic reflector with a focal length equal to about 0.3 timesthe diameter is to be preferred. If the illumination of the parabolic reector is tapered off towards the edge of the reflector, there is some lowering of the gain, but such a tapering off of illumination tends to reduce the amplitude of side lobes, which is an important consideration. In order to obtain a desirable radiation pattern for the entire system including the parabolic refiector, the parabolic reflector should therefore intercept a suicient solid angle of radiation to provide a somewhat tapered illumination. of the parabolic reiiector and the antenna feed sys'- tem should radiate very little laterally or in forward` directions. making substantial angles (such 11 as 15 or more) with the axis of the parabolic reflector.

The radiation pattern of a dipole antenna provided with a parasitic reflector dipole spaced onequarter-wave length away is usually represented by a curve such as is shown in Fig. 4. The position of the null corresponds to the direction towards the reflector dipole. The theoretical basis of this representation does not take into account any effect of the presence of the reflector dipole upon the behavior of the antenna dipole itself and it assumes that the reflector dipole radiates energy with a 90 phase difference with respect to the energy radiated from the antenna dipole. It is to be noted that if the reflector dipole is of an electrical length slightly different from that corresponding to a half-wave length, or if some reaction should take place between the two dipoles, the phase of the current excited therein and consequently the phase of the radiation from such reflector dipole may be different from that which would be expected from a reflector dipole of an electrical length of exactly a half-wave length and coupled only by radiation. The foregoing may explain the phenomenon that, as l have found, the spacing between antenna dipole and reflector dipole producing the best beam pattern in connection with a parabolic reflector should be of the order of one-eighth-wave length rather than one-quarter-wave length.

Experiments tend to establish, however, that the radiation pattern of the dipole pair arranged in accordance with this invention with approximately one-eighth wave length spacing between the dipoles is not of the type shown in Fig. 4,

but rather of another type, including a principal lobe of radiation and a small oppositely-directed additional lobe of radiation. Fig. 5 is a theoretical diagram of the calculated radiation pattern for a pair of dipoles spaced one-eighth wave length apart and excited with a phase difference of about 155, the intensity of excitation being equal for the two dipoles. The solid curve represents the pattern in a plane perpendicular to the dipole (the magnetic plane) and the dotted curve represents the pattern in the plane which includes the axes of the two dipoles (the electric plane). The dotted pattern is more directive since it, as calculated, corresponds to the modulation of the magnetic-plane pattern by the characteristic figure 8 radiation pattern of a single dipole in the electric plane.

Fig. 6 illustrates a typical experimentally obtained directive pattern measurement on a pair of dipoles comprising an antenna dipole and a reflector dipole located at one-eighth wave length from the antenna dipole, the dipoles being parallel. The solid curve represents the pattern measured in the magnetic plane and the dotted curve represents the pattern measured in the electric plane. The experimental pattern shown in Fig. 6 corresponds to the calculated pattern shown in Fig. 5 in the existence of sharp nulls in the broad side direction, in the more directive pattern in the electric plane and in the presence of a small lobe in a direction opposite to that of the principal lobe. The possible presence of additional factors not accounted for in the calculated curve is, however, suggested by the irregularities in the experimental curve and in the increased directivity of both the electric and magnetic plane patterns. It is possible that the thickness of the dipoles may be one such factor.

As a result of the small forward lobe of the vfeed system (the principal lobe being directed 'lengths or even less.

backwards towards the parabolic reflector), the maximum forward gain of the entire antenna system including the parabolic reflector varies rather considerably with the distance between the effective center of radiation and the vertex of the parabolic reflector, and this variation is periodic and provides a maximum forward gain when the aforesaid distance is equal to an integral number of half-wave lengths. This variation in gain is particularly noticeable with parabolic reflectors of small diameter, such as eight wave lengths or less and amounts to a factor of two for a reflector diameter of three wave lengths. The adjustment of the distance between the effective center of radiation and the parabolic reflector in accordance with this invention to produce maximum gain brings the radiation from the forward lobe into phase with the radiation reflected from the parabolic reflector.

A similar direct-forward lobe (sometimes called a back lobe, with respect to the feed) has been found tc,be exhibited by systems in which the auxiliary reflector is a sheet metal disk about a quarter-wave length in front of the antenna dipole. This may result from the presence of currents near the edges of the disk. The half-wave point location of the effective center of radiation is therefore desirable for such antenna systems, just as it is in the preferred antenna system having a reflector dipole about one-eighth wave length in front of the antenna dipole.

The forward radiation, when properly phased as above described by the location of the effective center of radiation at a half-wave point, appears to have an additional improving effect upon the directive properties of the entire antenna system resulting from the fact that the direct-forward lobe destructively interferes with the first side lobe of the radiation reflected from the parabolic reflector. The beam formed by the parabolic reflector is narrower than the direct-forward lobe and has a number of side lobes, some of the closer which fall within the angle occupied by the direct-forward lobe. It is known from principles of optics that the first side lobe on each side of 'the main beam will be opposite in phase to the main beam and that these two side lobes will usually be the most intense of all the side lobes. The direct-forward lobe, thus, at the same time that it adds to the main beam, counteracts the most troublesome side lobes, thus improving the overall directive qualities of the beam.

The amount of defocusing resulting from placing the effective center at or near a half-wave point should preferably be kept at a minimum. If possible it would be desirable to provide a parabolic reflector with a focal length equal to an integral number of half -wave lengths.

Antenna systems constructed according to this yinvention are particularly suitable for steerable directive systems employed for radio echo detection and location service. They may be used either for transmission or reception and indeed 'also for systems in which transmission and reception is accomplished with the same antenna system. Antenna systems according to this invention provide highly directive beams with narN row beam widths and extremely low side lobe radiation in spite of the fact that the parabolic 'reflector may have a diameter of only 4 wave This is a very great advantage for echo detection and locating systems for it is evident that the parabolic reflector,

13 which according yto Athe -'preferred fforrn of lconstruction itself supports the feed line fand antenna system, 'may "be 'mounted --in 'such 'Way that it may `berotated about twofdifierent axes, preferably 'at right angles lto 'each other. The mechanical 'arrangements'of ysuch fm'ountingg are well known as they 'have been =use`d `Afor guns, Searchlights and other 'devices fora long time. It is readily apparent lthat rotation aboutboth mounting axes can fbe accomplished s'in'lultaneously'to producea scanning of adesiredarea or, more precisely, of adesired-solid "angle, just as scanning by rotation about two axes has been 'performed in the past inthe `c'asef'of optical systenis (e. g., for ltelevision),"lawn sprinklers, 'and so on. The antenna system -of -the 'present 3invention lends itself readily Vto Iconstruction iin .a form `suiiicientl-y sturdy tovenable relatively `.rapid .rot/ation or other movement of the entire antenna system. If the antenna systemlandffeed-is'rotated together with v:the parabolic reflector, of course, some Erotating 'joints will be :necessary lin the yfeed :line which .connects the antenna with the transmitting and/orreceivingsyste'm. Such joints arelnow knownandfhave been successfully operated. The 'arrangement'of the 'feed lineiand the vprovision 'of .'-such joints, although lit may fbe vlan essential part'of a continuouslyiscanningltype of y.radio-echo v'detection and .location 'apparatus is not of direct concern "to the practice or the understanding of this invention, fand :for that reason is not described :in :detail '.here, :since .it .does not affect the .formation "of a .sharply `directive `beam 'ofi-radiation. :It is,however, .to be understood that 2antenna .systems according :to this invention have advantagesiparticularly suited for operation in '-a continuously scanning .ty-pe of 'detection and location apparatus. This is especially true with respect .to .the provision of low side lobe radiation, for sidesloberesponsein .a continuously scanning system 'which employs a vcathode ray type of .indicatordntroduces not only distracting signals 'but also an 'undesired Aambiguity because Vthe :indication resulting from the side lobe re'sponse Acannot be distinguished .from an indication produced by :the `mainibeam of radiation when pointed in .quite another `di .rection.

What Il'clesire.to1claim and 'secure by Letters rPatent is:

Vl. `A directive lantenna :system including adiipole :antenna connected to a two-conductorfee'd .line 'adapted :for connection `torrauiio apparatus,

a reflector Iparallel to said 'antenna .land `located rfow/arrflly thereof 'at i a distance not-substantially greater than 0.27 ywave length, .and a paraboli'c .reflector associated with 'said v.antenna and `located rearwardly 'thereof such a im'anner 'that Itho effective center of radiation defined .by 'cooperation of saiddipole-fantennaand said relflector lies approximatelyfon the'a'nisiof .said parabolic 'reflectorat'a distance from 'the vertex "of said parabolic 'reflector equal :approximately Ito 2. A directive antenna system including a `dipole antenna vconnected *to -a twoec'o'nductor feed line adapted for "connection to-radio apparatus,

va vreflector parallel `to 'said lantenna and located forwardly thereof at a distance y'not :substantially greater than 0.2'7 4Wave length, and a parabolic reflector having a focal length approximately "equalto 'an integral numberofhalflwave lengths 114 and 'located rearwardly fof said '-.antenna in such a manner that the effective fcenterfo'f radiation dene'd by thecooperation o'f saiddipoleantenna and said "first named reflector rlies at the focus of vsaid:parabolic reflector.

3.'A directive antennasystem .including a idipole antenna connected to va two-conductor vfeed line adapted for connection'to a radio 'fapparatua a'reect'or dipole parallel to'said antenna andf-located forwardly thereof atzatdistancenot greater than one-quarter wave length, and a paraboliczrevflectorhaving a focal lengthI approximatelyequal to an integral number of half-wave lengths and located rearwardly of said vantenna in such a manner 'that `the vmidpoint between the respective centers of said dipole antenna and said 1reflector dipole 'lies-at lthe focus of `said Lparabolic reflector.

4. A directive antenna system includingraidipole antenna connected to a two-conductor feed line adapted for connection to 'radio apparatus, a 1reiiector dipole parallel tosaid antenna and located forwardly thereof at azdistance substantiallyzless than one-quarter wave length, and a parabolic reflector associated 'with said antenna .and `1ocated rearwardly thereof in such-a 'manner that the midpoint between the 'respective centers fof said dipole antenna and said lreflector dipole is located approximately'on theaxis of said ,para-- bolic reector and-at a distance from the vertex of said parabolic reflector of :approximately an integral number of half-"wave lengths, .the said integral number lof half-'wave lengths being so chosen as to bring saidniidpoint as nearfas possible'to'the focus `of said-parabolic'reector under the conditions vherein defined.

5. A directive@ antenna system includingy a dipole antenna `connected to a two-conductor feed line adapted for connection to radio apparatus, a 1reector dipole parallelto said antenna an'dflocated forwardly thereof ata distance of approximately 'one-eighth wave length, and a parabolic reflector associated with said antenna having a focal length of approximately an integral 'number 'of half-wave lengths and'located `rearwardly of .said

antenna in such a mann'er'that the Amidpointbetween the respective centers-ofsaiddipole antenna and said reflector dipole lies atthe focus of said parabolic reflector.

6. A directive'antennal system including a dipole antenna connected to a coaxial-'conductor 4feed line adapted for connection to radio apparatusa sleeve choke'mounted on theouterconductor of :said'feedline withits openend located near .said dipole Yantenna'but not close venough thereto to 'permit the occurrence of corona discharge, a Arenector dipole parallelto said antenna and located -forwardly thereof at a .distancesubstantiallyless than one-quarter wave length and a iparabolic reflector associated with said antenna and lo- 'cated rearwardly thereof vin such a manner that said feedline passesthrough said parabolic re- 7iiector at the vertex of the said .parabolic vre1iec 'tor and is supported at said vertex and .further .that the midpoint'between the respective centers of said dipole antenna and said krefle'ctordipole lies approximately on the axis of said vparabolic yreflector at a distance'lfromsaidvertex equal'to an integral number of half wave .lengths and at a distance fromthe focus of said parabolic reflector which is not greater fthan one-half wave length.

7. A directive antenna system including adip'ole antenna having armsof'bulbous yshape connected I"to a co axial-conductor feed vline adapted Ifor connection to radio apparatus, a reflector dipole slightly longer than said dipole antenna, parallel to the axis of said antenna and located forwardly thereof at a distance substantially less than onequarter wave length, a sleeve choke mounted upon the outer conductor of said coaxial feed line having its open end near said antenna but not so near thereto as to permit the occurrence of corona discharge in the operation of said system and a parabolic reflector associated with said antenna and located rearwardly thereof in such a manner that the midpoint between the respective centers of said dipole antenna and said reflector dipole is located approximately on the axis of said parabolic reflector at a distance from its vertex of approximately an integral number of half-wave lengths and at a distance from the focus of said parabolic reflector not greater than one-half wave length.

8. A directive antenna system including a dipole antenna having arms of bulbous shape connected to a coaxial-conductor feed line adapted for connection to radio apparatus, a reflector dipole longer than said dipole antenna, parallel to the axis of said antenna and located forwardly thereof at a distance substantially less than one-quarter wave length, a sleeve choke mounted on the outer conductor of said feed line with its mouth directed towards said antenna, a prospective vessel enclosing said dipole antenna, said reflector dipole and said sleeve choke and made of material essentially transparent to radiation in connection with which said antenna system is adapted to operate, the space enclosed by said vessel co-mmunicating with the space between the conductors of said coaxial conductor feed line, said vessel being adapted to maintain pressure in the neighborhood of said antenna, and a parabolic reflector associated with said antenna and located rearwardly thereof in such a manner that the midpoint between the respective centers of said dipole antenna and said reflector dipole is located approximately on the axis of said parabolic reflector at a distance from its vertex of approximately an integral number of half-wave lengths and at a distance from the focus of said parabolic reflector not greater than one-half wave length.

9. A directive antenna system including a dipole antenna connected to a coaxial-conductor feed line adapted for connection to radio apparatus, a reflector dipole parallel to said antenna and located forwardly thereof at a distance substantially less than one-quarter wave length, a vessel of material essentially transparent to radiation in connection with which said antenna system is adapted to operate, which vessel encloses said dipole antenna and said reflector dipole and is sealed to the outer conductor of said coaxial-conductor feed line, said vessel being adapted to maintain an internal pressure and having an extended neck adjacent to said outer conductor of said feed line, means mounted on said outer conductor of said feed line covering said extended neck in such a manner as to constitute a sleeve choke resonant at the frequency of operation of said antenna system and at the same time to secure mechanically theA said vessel upon said outer conductor, and a parabolic reflector associated with said antenna and located rearwardly thereof in such a manner that the midpoint between the respective centers of said dipole antenna and said reflector dipole is located approximately on the axis -of said parabolic reflector at a distance from its vertex approximately equal to an integral number of half-wave lengths and at a distance from its focus not greater than one-half wave length.

10. In a directive antenna system which includes an antenna and a coaxial-conductor feed line connected to said antenna, a vessel enclosing said antenna and sealed to the outer conductor of said feed line made of material substantially transparent at the frequency of operation of said antenna adapted to maintain internal atmospheric conditions and having a neck extension adjoining said feed line and means mounted on the outer conductor of said feed line embracing said neck extension to define, in cooperation with said neck extension and a portion of said outer conductor, a choke resonant to the frequency of operation of said antenna and adapted to inhibit the formation of standing waves on the outside of said feed line, said means being also adapted to cooperate to secure said vessel in fixed relation to said feed line.

11. A directive antenna system including a dipole antenna connected to a two-conductor feed line adapted for connection to radio apparatus, a reflector dipole parallel to said antenna and located forwardly thereof at a distance substantially less than one-quarter-wave length, a protective vessel enclosing said antenna and said refiector dipole sealed to and mounted on said feed line, a support attached to said vessel for supporting said reflector dipole and for maintaining its position relative to said antenna, and a parabolic reflector associated with said antenna and located rearwardly thereof in such a manner that the midpoint between the respective centers of said dipole antenna and said reflector dipole lies approximately on the axis of said parabolic reflector at a distance from its vertex of approximately an integral number of half-wave lengths and at a distance from its focus not greater than one-half-wave length.

12. A directive antenna system including a dipole antenna connected to a coxial conductor feed line adapted for connection to radio apparatus, a transformer in said feed line near said antenna for improving energy transfer between said line and said antenna which comprises a portion of said line of approximately a quarter-wave length having a characteristic impedance different from that of the rest of said line,a sleeve choke mounted on the outer conductor of said feed line with its lopen end located near said dipole antenna but not close enough thereto to permit the occurrence of corona discharge, a reflector dipole parallel to said antenna and located forwardly thereof at a distance substantially less than one quarter-wave length and a parabolic reflector associated with said antenna and located rearwardly thereof in such a manner that said feed line passes through said parabolic reflector at the vertex o fthe said parabolic reflector and is supported at said vertex and further that the midpoint between the respective centers of said dipole antenna and said reflector dipole lies approximately on the axis of said parabolic reflector at the vertex of the said vertex equal to an integral number of half-wave lengths and at a distance from the focus of said parabolic reflector which is not greater than onehalf-wave length.

13. A directive antenna systemv including a dipole antenna having arms of bulbous shape connected to a coaxial-conductor feed line adapted for conection to radio apparatus, a transformer in said feed line near said antenna for improving energy transfer between said line and said antenna which comprisesa portion of said line of approximately a quarter-Wave length having a characteristic impedance different from that of the rest of said line, a reflector dipole longer than said dipole antenna, parallel to the axis of said antenna located forwardly thereof at a distance substantially less than one-quarter-wave length, a sleeve choke mounted on the outer conductor of said feed line with its mouth directed towards said antenna, a protective vessel enclosing said dipole antenna, said reflector dipole and said feed choke and made of material essentially transparent to radiation in connection with which said antenna system is adapted to operate, the space enclosed by said vessel communicating with the space between the conductors of said coaxial conductor feed line, a support associated with said vessel for positioning and supporting said reflector dipole, and a parabolic reflector associated with said antenna and located rearwardly thereof in such a manner that the midpoint between therespective centers of said dipole antenna and said reflector dipole is located approximately on the axis of said parabolic reflector at a distance from its vertex of approximately an integral number of half-Wave lengths l'and at a distance from the focus of said parabolic reflector not greater than one-half-wave length.

14. A direct antenna system including a dipole antenna having arms of bulbous shape connected to a coaxial-conductor feed line adapted for connection to radio apparatus, a transformer in said feed line near said antenna for improving energy transfer betwen said line and said antenna which comprises a portion of said line of approximately a quarter-wave length having a characteristic impedance different from the rest of said line, a reflector dipole longer than said dipole antenna, parallel to the axis of said antenna and located forwardly thereof at a distance substantially less than one-quarter-wave length, a sleeve choke mounted on the outer conductor of said feed line with its mouth directed towards said antenna, a protective vessel enclosing said dipole antenna, said reflector dipole and said feed choke and made of material essentially transparent to radiation in connection with which said antenna system is adapted to operate, the space enclosed by said vessel communicating with the space between the conductor of said coaxial conductor feed line, a support associated with said vessel for supporting and positioning said reflector dipole, and a parabolic reflector associated with said antenna and located rearwardly thereof in such a manner that the midpoint between the respective centers of the said dipole antenna and said reflector dipole is located approximately on the axis of said parabolic reflector at a distance from its vertex of approximately an integral number of half-wave lengths and at a distance from the focus of said parabolic reflector not greater than one-half wave length, said parabolic reflector having a focal length approximately equal to 0.3 'times the diameter of said reflector.

15. A directive antenna system including a dipole antenna conected to a two-conductor feed line adapted for connection to radio apparatus, a reflector parallel to said antenna and located forwardly thereof at a distance not greater than one quarter wave length, and a parabolic reflector associated with said antenna and located rearwardly thereof in such a manner that the effective center of radiation defined by the cooperation of said dipole antenna and said reflector is located at a distance from the vertex of said parabolic reflector equal approximately to an integral number of half Wave lengths, said integral number of /half wavelengths being chosen to bring said effective center nearest to the focus of said parabolic reflector.

LESTER C. VAN ATTA.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,054,896 Dallenbach Sept. 22, 1936 2,239,724 Lindenblad Apr. 29, 1941 2,407,057 Carter Sept. 3, 1946 2,413,187 McCurdy et al Dec. 24, 1946 Certificate of Correction Patent No. 2,486,620 November 1, 1949 LESTER C. VAN ATTA It is hereby certiied that error appears in the printed specication of the above numbered patent requiring correction as follows:

Column 4, lines 54 and 55, for transfoarmer read transformer; column 15, line 29, for the Word prospective read protective; column 16, line 59, for o fthe read of the; line 64, strike out the vertex of the and insert instead a distance from; column 18, line 19, for conected read oormeoted;

and that the said Letters Patent should be read ascorrected above, so that the same may conform to the record of the case in the Patent Oice.

Signed and sealed th1s 3rd day of July, A. D. 1951.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.